Tuning For Mileage

The following article, written by mpgmike, appeared on FuelEconomyTips.com website. It was originally posted as 3 articles, Tuning For Mileage Parts 1-3. I have combined them into one page and posted it here as a convenience and reference for my site's visitors. It is reprinted with the author's permission.

Tuning for Mileage by Mike Holler (mpgmike)

So you installed the Acme Fuel Mizer, the Presto Mileage Maestro, Sparky plugs, and a few other devices all claiming up to a 30% increase in mileage, but you are only seeing maybe a 10% gain. There are many devices and technologies on the market and the internet that are based on sound science that can’t seem to deliver the goods.

It has become painfully obvious by observing my apprentices on the mpgResearch Forum that a comprehensive Guide to Tuning is desperately needed. Simply making combustion more efficient these days isn’t enough. The factory ECU is programmed for the factory hardware. Once you deviate from that basic recipe, the ECU is no longer able to deliver optimal results. The fix? Tuning!

Tuning a stock vehicle usually won’t deliver much of an increase in mileage. Up to 20% gains have been reported, but typically fall into the 10% or less range. Once you add something to improve combustion efficiency, much larger gains are common. In fact, I’ve been seeing over 100 MPG regularly with Brown’s Gas, fuel heaters, vaporizers, ozone, and other devices, almost always in combination.

Let’s break the tuning process down into bite-sized steps. A logical format makes the tuning process more like science and less like a mystical black art.

1- Verify the vehicle is in good working condition
2- Install your mileage device(s)
3- Lower your lean-out limits
4- Adjust your air/fuel mixture
5- Adjust your ignition timing
6- Readjust your air/fuel mixture

Verify Vehicle Condition

A common cause of vehicles fighting mileage gains is a hidden problem with the vehicle itself. Tired oxygen sensors, clogged EGR circuits, carboned throttle bodies, ignition components that are not up to par, partially clogged catalytic converters, defective sensors, and a whole host of other problems have been found. Usually the vehicle runs perfectly fine, no codes are set, and the stock mileage is typical for that type of vehicle. The owner assumes that the vehicle is in top operating condition because he/she has no reason not to.

When the proper tuning procedure is followed and mileage gains just won’t come, go back and start nit-picking the vehicle apart. Check everything. You might even consider planning on a complete tune-up at this time. In fact, this would be a good time to upgrade to Pulstar Plugs, MPG Plus ignition wires, Blue Streak or Neihoff cap and rotor. Clean out your throttle body and PCV system. Install new filters and oxygen sensor. Make sure the basics are in order.

Install Mileage Device

The next step is to install your mileage device(s). Many people like to install upgrades one at a time to determine the overall effect each addition yields. Some like to just toss several on at a time. Usually finances dictate the one-at-a-time method. Be sure to install the device(s) properly. If it is a product you have purchased, follow the manufacturer’s instructions to the letter. If it is a device you have built from plans, perhaps from the internet, again, follow the instructions implicitly.

Any other modifications called for in the instruction manual should also be done at this time. Some devices require other changes in order to be effective. Without the other changes, the inventor cannot guarantee the results you seek. Short cuts usually short-cut your results.

Lower Lean-Out Limits

The ECU has parameters that it will not go beyond. The parameters that are correct for your modified vehicle almost always fall outside the range the ECU is prepared to operate. Combustion efficiency enhancing technologies will easily take your maximized operating conditions beyond what the ECU will tolerate. The solution is to change the parameters.

The ECU works similarly to our brains. It uses multiple inputs and controls multiple outputs. We have our 5 senses: hearing, sight, smell, taste, and touch. Within each of these senses there are a range of different inputs possible. The ECU has its senses as well: MAP (Manifold Absolute Pressure), TPS (Throttle Position Sensor), MAF (Mass Air Flow), ECT (Engine Coolant Temperature), IAT (Intake Air Temperature), Tach, O2 (oxygen sensor) and other inputs.

If the MAP sensor sees a given load, the TPS sees a corresponding throttle angle, the CTS sees a normal operating temperature, and the O2 sensor is saying the engine is too rich, the ECU will comply…to a point. When the ECU has leaned out the AFR (Air/Fuel Ratio) beyond what the programming claims is an acceptable range, the ECU will go into Open Loop and ignore the O2 sensor. It then reverts to Look-Up tables for its source of information. At this point, mileage will invariably go down, and often a trouble code is set.

Consider the conditions needed for the ECU to accept lean fuel commands. If the engine is warmer than it actually is, the ECU will accept leaner. If the engine is under less of a load, the ECU will want to deliver less fuel. If the incoming air is hotter, the ECU will accept lean commands more readily. If MAF (Mass Air Flow) sensor equipped, less air entering the engine will require less fuel.

Now let’s look at the particulars.


One of the easiest ways to lower lean-out limits is to install a resistor across the CTS and IAT sensors in parallel with the sensor. A parallel circuit offers 2 paths of travel for the voltage. A cold CTS will have very high internal resistance. As it warms up, the resistance goes down. Adding a parallel resistor nets a lower total resistance value, thus sending a hotter temperature signal to the ECU. It should be noted that this trick applied to the IAT sensor will retard ignition timing in addition to lowering the lean-out limits.

Diagram of series vs parallel connection

Most of the world uses similar resistance values to equate a given temperature. The Ford based systems (including Mazda, Infiniti, and Jaguar) use much higher resistance values. This is important to know when selecting resistors. If you have a scan tool available to you, use it. Monitor the CTS temperature that the ECU sees. With your engine at operating temperature, check to see that the temp reading is close to the thermostat rating. If it is, proceed. If it isn’t, check your cooling system for contamination or stuck thermostat. You may need to do a coolant flush or repair before proceeding. Assuming you are getting reasonable numbers, try different resistors across the CTS to raise the temp reading about 10° F (for example, from 195° to 205°). Even though higher numbers will work, you will most likely run into cold start issues beyond the 10° offset. The average vehicle will use something like a 3.9K ohm resistor. Fords may like a 5K ohm or larger value.

Editors note: Many people have successfully used a pot (or potentiometer) instead of a fixed resistor. You can find these at Radio Shack or other electronics supplier. This allows you to adjust the resistance to the exact amount you need and eliminates a great deal of trail and error.

If your cooling fan runs continuously with your setting, add more resistance to lower the temp reading. Any mileage gains from the hotter engine signal will be more than offset by the additional load on the alternator. If you have a rear-wheel-drive with a belt driven fan, you can still add too much temperature offset. The ECU has an internal cooling mode. After the engine overheats to a point, the ECU starts dumping copious amounts of fuel. The excess fuel will evaporate, thus cooling the engine from the inside. However, at this point your mileage literally tanks.


The IAT is less sensitive to cold start issues. You can add more temp to this signal than you can to the CTS. Just keep in mind that you are not only lowering your lean-out limits, you are also retarding your ignition timing. If you put a timing light on the engine as you adjust IAT values, you won’t see the timing change. The timing changes under load. Hotter air is more prone to detonation. This is why the ECU retards the timing.

If you are tuning on the hottest day of the year, you may find out just how high of a signal you can generate before setting codes. Typically it is in the 240° F range. If you are tuning in the middle of February, then you can offset the signal from your base cold reading and things will be fine for now. Come June or August, this setting may be high enough to trip codes. Allow for this when tuning.


It is important to address your load sensing system in order to keep your mileage gains. Often times addressing only a few of the ECU’s inputs will allow you to tune for a mileage gain, only to have the adaptive memory take it away as time goes on. The load sensing devices give the ECU a clue as to what you are up to, and it won’t tolerate it. By generating a lower voltage signal from the MAP sensor, harmony is restored, and your mileage gains are for keeps.

There are 2 types of MAP sensors on the market. Most of the world uses a version that has a 5 volt VREF, ground, and DC signal wire. The MAP is a type of potentiometer; like a radio volume knob. Instead of turning the knob with your hand, the knob is turned as the vacuum in the engine changes. A high vacuum reading will give a low voltage signal. A low vacuum reading will give a high voltage signal. Low vacuum means the engine is under load and needs lots of fuel. Look at it this way, low signal voltage, low fuel requirements. High signal voltage, high fuel requirements.

If you raise the VREF, then the signal will be higher. If you lower the VREF, then the signal will be lower. A lower signal tells the ECU lower load. A relatively simple method of lowering the VREF is with an LM317T adjustable voltage regulator. If you are somewhat capable with electronics, you can build one for about $10 to $15. The parts you will need are:

- LM317T adjustable voltage regulator
- 220 ohm resistor (1/4 watt is sufficient)
- 1K ohm multi-turn potentiometer (Trim pot)
- Small heat sink for the LM317T
- 3 different colors of 18 gauge wire
- Enclosure (box)

MAP sensor handler diagram

The LM317T comes in a TO-220 case. Looking at it from the front with the mounting tab at the top and the 3 pins at the bottom, solder the 220 ohm resistor across the 2 left pins. Run a jumper wire from the left pin to the center of your 1K pot. Run a ground wire to one of the outside legs of the pot. If you use one side, then clockwise will raise the voltage. If you use the other side, then counter-clockwise will raise the voltage. Bench test your unit to know which way it will work.

Run the right leg to a Key-On/Crank battery voltage source. It is important that you have voltage when the key is on AND when cranking. If you don’t have voltage when cranking, the ECU will not see a MAP signal and usually won’t start at all. Drill 2 holes in your enclosure; one for the wires and one to access the pot.

With your unit on the bench, apply battery voltage to the battery and ground leads. Check the voltage output. Adjust it to 5.0 volts to start with. To install it on your vehicle, cut the VREF wire going to the MAP. DO NOT TAP THE VREF WIRE COMING OUT OF THE ECU! This will affect all sensors using the same 5 volt signal and will deliver disastrous results. After you cut the wire, connect your adjusted voltage wire to the MAP sensor and tuck and tape the other end of the cut wire back into the harness. It is best to solder connections and seal with heat-shrink tubing.

Lowering the VREF voltage will lower your lean limits, but will also advance ignition timing. Less load equals more timing advance. More load equals less timing advance. Remember this when we get to step 5.

The other type of MAP sensor used almost exclusively on the Ford based systems is frequency based. It has a 5 volt VREF, ground, and frequency signal output. This type of MAP or MAF will not work with the handling above. FuelSaver-MPG Inc. will soon have a frequency based MAP/MAF enhancer that will do this job for you. Editor's note: We have removed a section from this document that recommended inserting a resistor on the ground wire as a handling for frequency based MAP/MAFs. We found that this was not workable, and have deleted it. If you want to try it, the delected text is as follows: Simply cut the ground wire going to the sensor, then add a small amount of resistance. A good starting point is about 10 ohms. Your upper limits will be between 15 and 20 ohms, depending on the ECU’s calibration. The more resistance, the more offset. Notice I didn’t say 10K ohms. It takes very little resistance to accomplish the job. You may find an Ohm Ranger or small value potentiometer helpful here.


There are a couple different styles of MAF sensors that have been employed over the years. Early versions were called Vane Air Flow (VAF) sensors. They had a spring loaded door that controlled a wiper arm across a resistive pad. There is a black plastic cover that, once removed, will allow access to this resistive circuit. Raising spring pressure will lower lean-out limits. You can shift the wiper arm to a clean spot on the resistive circuit to extend the life of your VAF while you’re in there.

Some of the MAF sensors work like the typical MAP sensors in that they have a DC voltage IN, and a DC voltage signal OUT. They can be dealt with the same as the typical MAP sensor.

Most of the modern MAF sensors have a ground, battery voltage input, and frequency based output. These aren’t that difficult to tune. I’ve seen complicated and expensive products that aren’t much better than this trick. Just like the frequency based MAP sensors, cut the ground wire and install a small amount of resistance. Again, no more than 30 ohms, with averages in the 10 to 15 ohm range.

Adjust AFR

Once you have set the stage by lowering your lean-out limits, you can now adjust the AFR for better mileage. The exact method will depend on the type of O2 sensor your vehicle uses. There are 4 types currently on the market: old style oxygen sensor, AFR sensor, titania sensor, and wide-band sensor. Each requires its own unique approach. Some vehicles may have 2 bank sensors. You’ll have to address both equally. Don’t worry about the downstream sensors (the ones after the cat) as they only tell the ECU that the converter is working.

To do your adjustment, you want to monitor your Loop Status. If your ECU pops into Open Loop, any adjustments you make are irrelevant. You have to stay in Closed Loop. If you pop into Open Loop, your lean-out limits are still too high (or your oxygen sensor is bad). If you can install a scan tool to monitor Loop Status, do it. This is the easy way. If you have an older vehicle that doesn’t have data stream information, then hook a digital volt meter to the O2 signal wire. As long as the voltage is jumping around, you’re probably in Closed Loop. If the voltage goes steady, you probably are in Open Loop.

As you begin to lean out the mixture you will probably feel an increase in power. There will be a peak in the power, then it will gradually taper off. After so much leaning out, it will be like you just fell off a cliff. There will be a dramatic loss in power, it will want to hesitate and stall. I try to tune about ½ way between peak and cliff.

Old Style O2 Sensor

The oxygen sensor was introduced in mass back in 1981 on GM vehicles. It has an operating range of 0-1 volt. The higher the voltage the richer the detected AFR. The lower the voltage, the leaner the AFR. A rich mixture is a lean command. A lean mixture is a rich command. It is commonly called a Narrow-Band sensor because it is only accurate within a narrow range of AFR operation. Right at the 14.7:1 AFR, a small change in AFR yields a large change in voltage. As the engine goes leaner or richer from the 14.7:1, the voltage changes get smaller and smaller.

A device that has been used for several years is the Electronic Fuel Injection Enhancer (EFIE) developed by George Wiseman of Eagle Research. The principle is to create a small amount of voltage offset that is electrically isolated from chassis ground. It is like a free-floating battery installed inline with the signal wire. This raises the voltage to the ECU indicating a richer-than-actual AFR.

If you have an older vehicle with loose parameters, you may be able to add as much as 0.450 volts to the O2 signal. If you have a newer vehicle with tight parameters, you may not be able to get away with more than about 0.280 volts. Experimentation will dictate what your ECU will tolerate.

The old single wire sensors are easy to spot the signal wire, since it is the only one. There have been 2-, 3-, and 4-wire sensors used over the years. You may have to use a manual to determine which wire is the signal wire. Usually on a 4-wire sensor, you have 2 white wires for the heating element, a grey wire that is ground, and a black wire for your signal out.

Another method that I have not personally tried, but comes with good recommendations is to drill out a spark plug anti-fouler for the mid-70s Ford products with the 18 mm plug. Install the modified anti-fouler into the exhaust where the O2 sensor normally goes, then screw the O2 sensor into the anti-fouler. This pulls the sensor out of the exhaust stream and allows for leaning out the AFR. Since I haven’t tried it, I cannot guarantee results.

Wide Band Sensors

Good news! You can use the same EFIE on the signal wire of a wide band. The one wide band that I modified used the blue wire for the signal. Wide bands will have 5 wires. That’s the dead give-away. They have been used widely on VWs and Mazdas.

AFR Sensors

AFR sensors operate under a totally different set of rules. The same sensor is used in 2 different ways by various OEMs. One method involves putting a fixed voltage on the reference wire (white) and varying the current to maintain a fixed voltage on the signal wire (blue). Another method is to apply a fixed voltage and current to the reference wire, and monitor the voltage coming out. Either way, they are current devices, not voltage devices.

To alter an AFR sensor, cut the blue signal wire and install low value resistors. The range will be 30 ohms or less. Most of the vehicles I’ve modified have liked the 7 to 18 ohm range. I’ve never needed over 20 ohms as of yet. Again, an Ohm Ranger or low value pot will be helpful in your tuning. You will be able to feel 1 ohm resistance change.

Editors note: We have not had luck using a resistor on an AFR sensor. If you do this handling, please document your mileage before and after this one step, and if you have success, please contact me and let me know. We recommend doing the MAP and temp sensor handlings if you have AFR sensors.

Titania Sensors

They work similarly to the traditional O2 sensors, but backwards. I haven’t yet had the opportunity to deal with these, so I can only give you guesses on what will work. One possibility is to install an EFIE backwards so you are lowering the signal voltage. Another way might be to add resistance to the signal wire. The spark plug anti-fouler may also work. Fortunately, they were only used for about 3 years and only on select vehicles. See wiki.

Adjust Ignition Timing

If you have a distributor, the solution is simple. Loosen the hold-down clamp and turn the distributor. If you have DIS (Distributorless Ignition System), COP (Coil On Plug), or a distributor that doesn’t affect timing, then you have to play with the MAP and IAT signals to dial in the spark.

To adjust with the distributor, grab a vacuum gauge, timing light, and distributor wrench. Drive the vehicle down a relatively flat section of road at cruise speed. Watch your vacuum gauge. Pull over and either advance or retard the timing by about 4 degrees. If you increased the power and vacuum, adjust again by about 2 degrees. If you lost power and vacuum, crank it the other way about 8 degrees and test again. You want the least amount of timing advance needed to maintain maximum power at cruise. Any more advance than that will increase the possibility of detonation, and will fight the piston on the compression stroke.

If you have the DIS or COP, adjust the IAT sensor reading by 10° F increments for maximum power. If you start high on your reading (smaller value resistor), start adding resistance to advance timing. If you have a near ambient reading (no or large value resistor), reduce parallel resistance to retard timing.

As stated earlier, adjusting the MAP VREF will alter timing. A lower VREF will advance timing. A higher VREF will retard timing. There is a balance between finding the right lean-out limit, and maximizing the timing.

Readjust AFR

Improving combustion efficiency usually requires less timing advance to get the job done. The fuel burns faster and more thorough. Thus, it takes less time to convert the chemical energy in the fuel into kinetic energy at the crank. On the flip side, as you lean out the AFR, it takes longer to burn across the leaner mixture. More timing advance is required. Sort of a catch-22. Once you have adjusted your ignition timing for maximum power, you may find that you can lean out your AFR a bit more. A leaner mixture requires more time for the flame to propogate across the cylinder. If you advanced your timing, try leaning out the AFR a bit more. If you retarded your timing, you might be able to slightly enrich the AFR and get better power and mileage.


Tuning was taught in tech schools up until about 30 years ago. As the vehicles became more complex, there was less tuning required for optimal performance and mileage. Tech school grads coming into the work force these days are taught to follow flow charts, replace bad parts, and track down poor connections. For the younger generation, tuning is something they read about in magazines, but never get to experience. It isn’t difficult, but requires a learning curve just like anything else. Don’t get discouraged quickly. It may take a little time to get the hang of it, but you can do it.

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